CDP Manager - Detailed Documentation

Managing Vaults to be transferred between users

• Contract Name: cdpManager.sol

• Type/Category: Vault Management

• Associated MCD System Diagram

• Contract Source

• Etherscan

1. Introduction (Summary)

Summary: The DssCdpManager (aka manager) was created to enable a formalized process for Vaults to be transferred between owners, much like assets are transferred. It is recommended that all interactions with Vaults be done through the CDP Manager. Once unlocked collateral has been deposited into the Maker Protocol, users can make use of the following features:

• Multi Vault ownership and numerical identification (users can own N number of Vaults)

• Vault transferability

MCD System Diagram: Vault User and Vault Manager interaction diagram

Note: The MCD system diagram above shows that the Vault user goes through the proxy in order to interact with the CDP Manager but it is also possible to directly use the CDP Manager contract.

2. Contract Details

Key Functionalities (as defined in the smart contract)

• cdpAllow(uint cdp, address usr, uint ok): Allow/Disallow (ok) a usr address to manage the cdp.

• urnAllow(address usr, uint ok) : Allow/Disallow (ok) a usr address to interact with an urn for the purposes of either entering (src) or quitting (dst).

• open(bytes32 ilk, address usr): Opens a new Vault for usr to be used for an ilk collateral type.

• give(uint cdp, address dst): Transfers cdp to dst.

• frob(uint cdp, int dink, int dart): Increments/decrements the ink amount of collateral locked and increments/decrements the art amount of debt in the cdp depositing the generated DAI or collateral freed in the cdp address.

• frob(uint cdp, address dst, int dink, int dart): Increments/decrements the ink amount of collateral locked and increments/decrements the art amount of debt in the cdp depositing the generated DAI or collateral freed into a specified dst address.

• flux(bytes32 ilk, uint cdp, address dst, uint wad): Moves wad (precision 18) amount of collateral ilk from cdp to dst.

• flux(uint cdp, address dst, uint wad): Moves wad amount of cdp collateral from cdp to dst.

• move(uint cdp, address dst, uint rad): Moves rad (precision 45) amount of DAI from cdp to dst.

• quit(uint cdp, address dst): Moves the collateral locked and debt generated from cdp to dst.

Note: dst refers to the destination address.

Storage Layout

• vat : core contract address that holds the Vaults.

• cdpi: Auto incremental id.

• urns: Mapping CDPId => UrnHandler

• list: Mapping CDPId => Prev & Next CDPIds (double linked list)

• owns: Mapping CDPId => Owner

• ilks: Mapping CDPId => Ilk (collateral type)

• first : Mapping Owner => First CDPId

• last: Mapping Owner => Last CDPId

• count: Mapping Owner => Amount of CDPs

• allows: Mapping Owner => CDPId => Allowed Addr => True/False

3. Key Mechanisms & Concepts

Summary

The CDP Manager was created as a way to enable Vaults to be treated more like assets that can be exchanged. Originally, the dss core contracts did not have the functionality to enable transferring Vault positions. The CDP Manager was created to wrap this functionality and enable transferring between users.

High-level Purpose

• The manager receives the vat address in its creation and acts as an interface contract between it and the users.

• The manager keeps an internal registry of id => owner and id => urn allowing for the owner to execute vat functions for their urn via the manager.

• The manager keeps a double linked list structure that allows the retrieval of all the Vaults that an owner has via on-chain calls.

  • In short, this is what the GetCdps is for. This contract is a helper contract that allows the fetching of all the Vaults in just one call.

CDP Manager Usage Example (common path):

• A User executes open and gets a CDPId in return.

• After this, the CDPId gets associated with an urn with manager.urns(cdpId) and then join's collateral to it.

• The user can then execute frob to choose which dst address they want to use to send the generated DAI to.

• If the user executes frob without dst then the generated DAI will remain in the Vault's urn. In this case, the user can move it at a later point in time.

  • Note that this is the same process for collateral that is freed after frob (for the frob function that doesn't require the dst address). The user can flux it to another address at a later time.

• In the case where a user wants to abandon the manager, they can use quit as a way to migrate their position of their Vault to another dst address.

4. Gotchas (Potential source of user error)

• For the developers who want to integrate with the manager, they will need to understand that the Vault actions are still in the urn environment. Regardless of this, the manager tries to abstract the urn usage by a CDPId. This means that developers will need to get the urn (urn = manager.urns(cdpId)) to allow the joining of collateral to that Vault.

• As the manager assigns a specific ilk per CDPId and doesn't allow others to use it for theirs, there is a second flux function which expects an ilk parameter. This function has the simple purpose of taking out collateral that was wrongly sent to a Vault that can't handle it/is incompatible.

• Frob Function(s):

  • When you frob in the CDP manager, you generate new DAI in the vat via the CDP manager which is then deposited in the urn that the CDP manager manages. This process depends on which frob function you use (there exist two frob functions). In short, one allows a destination address and the other doesn’t require it.

  • If you use the frob function that has the destiny (dst) address, you are saying that you can send any Dai generated or collateral that has been freed. The second frob function is meant for leaving the collateral in the urn address because the urn is owned by the CDP manager. In this case, you would need to manually use the flux or move functions to get the DAI or collateral out. These functions (flux and move) may be more beneficial for a developer working with the proxy function, as it allows for more flexibility. For example, by using these functions you can move a specific amount of collateral and can use the other functions to do it. Overall, it can make working with it a little more flexible on specific developer needs.

• As mentioned above in the summary, the dss core contracts originally did not have the functionality to enable the transfer of Vault positions. Since then, the core contracts have also implemented a native transfer functionality called fork which allows the transferring of a Vault to another address. However, there is a restriction, which is that the address owner that will be receiving the Vault needs to provide authorization that they do in fact want to receive it. This was created for the situation when a user is transferring the collateral that is locked as well as the debt generated. If you are simply moving collateral to another address, there is no issue but in the case that you are also transferring the debt generated, there is a chance of putting a perfectly safe Vault in a risky position. This makes the contract functionality a little more restrictive. Therefore, the CDP manager is a good option to keep a simple way of transferring Vaults and recognizing them via a numeric ID.

5. Failure Modes (Bounds on Operating Conditions & External Risk Factors)

Potential Issues around Chain Reorganization

When open is executed, a new urn is created and a cdpId is assigned to it for a specific owner. If the user uses join to add collateral to the urn immediately after the transaction is mined, there is a chance that a reorganization of the chain occurs. This would result in the user losing the ownership of that cdpId/urn pair, therefore losing their collateral. However, this issue can only arise when avoiding the use of the proxy functions via a profile proxy as the user will open the cdp and join collateral in the same transaction.